A narrow band of very low, very gentle hills extends across the northern states from Cape Cod to the Rocky Mountains in Montana. In places the winds and rains of thousands of years have worn them down to insignificant undulations; in other places they may be a hundred feet high or more. There is nothing about it to catch the casual eye, but the geologist recognizes this ridge as the terminal moraine of North America’s last continental glacier, the line where the ice ended its long advance and began to melt back.

The glacier that stood on that line would have been a spectacular sight had there been anyone to see it: a great palisade of green and white ice many hundreds of feet high and stretching to the horizon cast and west. For the most part, it probably loomed silently menacing, but from time to time huge sections crumbled off in awesome avalanches. Forests grew, and herds of woolly mammoths and other, less lordly creatures grazed almost up to the ice face, but to the north, atop the glacier, there was only a barren expanse of blizzard-swept ice stretching in absolute desolation toward the Arctic Circle.

The glacier was the last cataclysmic event that helped shape the face of the continent. North of the line of terminal hills, the land was drastically changed. Rock was rasped from mountains till the craggy buttresses were smoothed away and the valleys ground wider and deeper. Soil and gravel were stripped down to bedrock in some places, and the land level dropped as much as hundreds of feet deep in others. Myriads of lakes were gouged into the land, while rivers were dammed, diverted to new beds, and sometimes even shunted bodily from one drainage system to another. The effects of the ice sheet are not only still very apparent, they are often so marked as to have influenced the course of our history and helped to shape the economic pattern of the northern part of the continent. They have even had a strong effect in molding the character of some of our people.

If that last statement sounds extreme, consider that most individualistic type, the New England Yankee, and note well his relationship to his land. When the ice sheet ground its way across New England, it scoured off most of the soil down to the granite ribs of the land—and when it melted back much later, it dropped millions of boulders over the landscape to make the region even less promising. It is not exactly a fat and hospitable land, and the people who elected to live there have had to spend their lives contending with it for a living in a battle so close that they could not help but absorb some of the flintiness of their own fields.

If the New England rocks yielded a scant living, they were an excellent hone against which the wits of the men who lived among them were sharpened to a fine edge, until “Yankee ingenuity” came to denote the ultimate in the ability to improvise and to get much out of little. At the same time, the streams that foamed through the uneven, glaciated hills provided abundant water power to put that ingenuity to work. As a result New England was, for a long time, the industrial heart of the nation.

What was the nature of this phenomenon that sent unbelievable quantities of ice grinding out of the north until more than half of North America was buried a mile or two deep? Contrary to popular belief, the ice did not form around the North Pole and then flow southward. It formed in a number of centers—Canada, Greenland, Europe, Siberia—more or less simultaneously, and spread from each of those places. Nor was the glacial era a period of unusual cold; the essential for a glacier’s formation is only that more snow shall fall during the winter than can melt in summer.

And times have come when the winter snow has outlasted the summer sun, not just for a few seasons but for thousands of years, piling up inch by inch, foot by foot, and packing into ice under its own weight until it came to be probably two miles or more thick at the center and hundreds of miles across. It was a burden so massive that it pushed the rock foundations of the land down into the molten magma on which they float. In many places the land is still springing back as the displaced magma flows back in currents of unbelievable ponderousness. The land in the Hudson Bay region, where the ice first started to accumulate and disappeared last, still has a long way to rise; it is estimated that eventually the bottom of the bay will emerge as dry land-provided another glacier does not come first.

As the ice became thousands of feet deep, the enormous pressure began to force the great mass outward at the edges. Meanwhile, the unending snows kept falling and turning to ice, always maintaining the pressure and keeping the glacier pushing forward, grinding rocks to pebbles and pebbles to clay and sand, and moving incredible quantities of rock and earth over the face of the land.

The glacier eventually reached its limit when weakening forward movement was just balanced by the rate at which the ice front was melting back, a period of equilibrium that probably lasted for hundreds of years, during which there were only minor fluctuations of the ice front. During this time, the glacier acted as a giant conveyer belt, carrying forward great quantities of boulders, gravel, and clay embedded in the ice, which were exposed and dropped at the melting front of the ice mass. In this way were amassed the great drifts, or moraines, that still trace the forward limit of the glacier.

The most easily identifiable stretch of moraine was laid down in the East, where the glacier stopped with its leading edge a number of miles offshore from the present coast. Much of the tremendous burden it dropped still abides, though considerably worn away by the sea; its largest portions are Long Island, Block Island, Martha’s Vineyard, Nantucket, and Cape Cod. Here, for what comfort it may give some Vermonter or New Hampshireman fighting poverty on a hardscrabble farm, is much of the earth that once clothed his hillsides.

To the west, the ice just buried Manhattan and stopped with an advanced lobe resting on Staten Island. Its line bent to the north in Pennsylvania because the mountains, while unable to stop it, did hold il back somewhat. It reached almost to the Ohio River in Indiana and Ohio, then tended in an irregular line northwestward until it came almost to the Canadian border in North Dakota, and then ran straight westward to the mountains. The western mountains were too high to be overrun, but they proliferated a rich crop of mountain glaciers that joined together into an almost solid icecap which connected with the continental glacier to the east.

There were other glaciers before the one we are concerned with, hundreds of thousands of years earlier. Some of them pushed farther to the south; dim remains of ancient terminal moraines show that ice has reached as far south as Louisville, St. Louis, and Topeka.

After pausing at its farthest-advance line, the glacier began to melt back. It was hardly a precipitous retreat, because a mile-deep ice mass does not fade away in a summer or two. During the period in which it was melting back through New England, the Connecticut Kiver valley was a lake and thus provided a sort of measured course over which we may time the rate of retreat. Each summer a thin layer of sandy sediment settled to the bottom; each winter an even thinner one of fine silt drifted down. Each pair of layers, called a varve, indicates the passage of a year just as plainly as does a ring on a tree. From a painstaking count of these varves, geologists find that it took more than four thousand years for the ice to melt from the present site of Hartford, Connecticut, to that of St. Johnsbury, Vermont, a sluggish average rate of only about one mile in twenty-two years.

But this, like all averages, is misleading because the retreat was not a steady one. Sometimes the ice melted back without interruption and dropped its load of debris in an even layer on the land. At other times the ice front would pause, balanced between its rate of melting and another time of forward movement, so that a ridge or “recession moraine” would be laid down. And occasionally a new forward surge would override and scatter earlier deposits.

So, both coming and going, the ice reshaped the land and left it far different from what it had been. Consider, for example, two of the many rivers it altered and rerouted. At one time the streams that now make up the headwaters of the Ohio flowed into Lake Erie; when the ice dammed the old courses, the water was forced to flow south and west around the glacier front until it found its way to a tributary of the Mississippi anil so formed part of the Ohio River system. In the West, the upper Missouri also followed a different path; it flowed north into Canada until the ice blocked that outlet and made it find a new channel that also led to the Mississippi.

These two river diversions are especially interesting because of their subsequent effects on American history. The first settlers across the Allegheny Mountains found rivers that led toward the setting sun; and they underwent a quick change in economic orientation because all at once it was easier to ship their produce all the way to New Orleans by flatboat than to haul it by wagon a few dozen miles back across the mountains. The Missouri performed an equally important function. It was the only water roule leading from the Mississippi into the western mountains at a point only a short distance across the continental divide from a river that flowed into the Pacific, it thus became the route of Lewis and Clark, and after them the surge of traders and mountain men who crossed to the Columbia River valley and so established the firm American claim to the Pacific Northwest.

The last continental glacier is only the latest of four which came and went during the Pleistocene epoch. Geologists sometimes refer to the era as the “last ice age” in recognition that there were other—and more severe—periods of glaciation back through the dim mists of time. The first of the four “recent” glaciations, called the Nebraskan, came an estimated one million years ago, and was followed in turn by the Kansan, Illinoian, and finally, the Wisconsin glaciations. Geologists have named them from the areas where their traces were most clearly identified and studied; the glaciers that occurred at the same time in Europe have completely different appellations. Between the times of ice, there were long interglacial periods when the climate grew mild, and most of the work of the glaciers was erased by a hundred thousand or more years of erosion.

Although scientists have generally agreed on a chronology of the comings and goings of ice, they are the first to admit that the dates are only the best possible estimates. But about one event there is little guesswork; we know almost exactly when the last glacier reached its southern limit. It so happened that a final surge of the ice overran a spruce forest near what is now Two Creeks, Wisconsin, snapping off the big trees and burying them deep under sand and gravel. Nuclear physicists, by a method recently discovered, are able to determine quite accurately the age of a bit of organic matter by finding what degree of radioactivity still remains in the carbon it contains. Complex Geiger counter tests of Two Creeks spruce proved that the forest had died approximately eleven thousand years ago, give or take a century or two. It was a result that confounded many scientists who had placed the peak of the glaciation at least twenty-five thousand years ago, but subsequent radioactive carbon datings from many sources have since corroborated the readings from the Two Creeks wood.

Eleven thousand years is not a long time, geologically speaking. The ice-torn land has mellowed in that time, but the softening is only superficial and our northern landscape is still basically a glaciated one. Just as the ice left the altered Ohio and Missouri rivers to show it had passed that way, it also produced many other monuments to its action.

The most spectacular of these is the Great Lakes. All four glaciers played a part in producing them, each one scooping the basins a bit deeper, until the last, the Wisconsin glacier, left their beds pretty much as they are today. The Great Lakes are the earth’s largest body of fresh water and comprise its greatest inland waterway. From the days of the French explorers they have been an avenue into the heart of the land, one which has become even more important with the opening of the St. Lawrence Seaway. It is the further good fortune of America that the lakes are rimmed with rich deposits of iron and coal, which have been laced together with cheap transportation to form the greatest industrial complex in the world.

When it formed the Great Lakes, the glacier also increased their usefulness by providing good access to them to supplement the St. Lawrence outlet. As it melted back, the glacier not only filled the lake basins but also formed an enormous dam to prevent excess water from draining across the lowlands to the north. The overbrimming water had only one place to go; it burst through to the south in several mighty torrents.

One of these was the great river that tore a channel from Lake Erie to the Hudson River. The Mohawk River now flows in the eastern part of the valley, which has been a strategic pathway between Lake Erie and the Hudson ever since the French and English started squabbling over North America. Its most important period, however, began in 1825 with the opening of the Erie Canal, which was dug in the bottom of the old glacial watercourse and brought a rush of settlers into western New York and onward into the Northwest Territory.

Another overflow channel left Lake Erie at the point where Toledo now stands, and ran into the Ohio. The Maumee and Wabash rivers now follow the same valley, which became the route of the almost forgotten Wabash and Erie Canal, between Toledo and Evansville, Indiana, whose 452 miles made it the longest in the country. Although a financial failure, it had a big part in opening northern Ohio and Indiana to settlement and development.

Still another outlet ran across the site of Chicago and, via the present courses of the Des Plaines and Illinois rivers, to the Mississippi. Between 1836 and 1848 the Illinois and Michigan Canal was dug in this valley to tie Lake Michigan to the Illinois River, and thereby to the Mississippi. Another waterway, usually called the Drainage Canal, was constructed much later along part of the same route to do double duty as a carrier of both Chicago’s sewage and water commerce to the Mississippi, but it did not supersede the old canal, which is still in use, a very rare survival of the heyday of canal building, (The Erie Canal, or its successor, the New York Barge Canal, cannot make the same claim because it was completely rebuilt and much of it relocated during this century.)

Although the Great Lakes are the largest glacier-created bodies of water, there are countless other lakes from the Dakotas to Maine and north into Canada. Minnesota alone has many more than enough to justify its tourist-attracting slogan of “Land of Ten Thousand Lakes,” while in Canada they are so numerous and interconnected as to be virtually uncountable, since it is a matter of individual judgment where one ends and another begins.

These are the waters that bore the bark canoes of the Chippewas and Crees, and later of the coureurs de bois. The forests around them have provided many fortunes in fur and timber; a very respectable amount of pelts and lumber still come from among them, but today the lakes are producing an additional bonanza: recreation dollars. The lakes are all creatures of the ice sheet, some gouged out of the earth or even out of rock, others created by glacier-built dams, but most simply formed when ice water filled the hollows and valleys in the churned-up mass of glacial debris.

But the greatest of the glacial lakes is long since gone. Geologists call it Lake Agassiz, after the Swiss-American who pioneered in advancing, and securing acceptance for, the ice-age theory; at its greatest extent it filled the valley of the Red River of the North in Minnesota and North Dakota and covered most of Manitoba—an inland sea larger than all the present Great Lakes combined. It formed when the retreating glacier left the area clear, while still blocking the normal northward drainage of the Red River; the resulting lake grew larger as the ice melted back, until the time came when the glacier withdrew so far that the water could rush out into Hudson Bay.

The only remnants of Lake Agassiz are Lake Winnipeg and some swamps and smaller lakes, but its ancient bed is easy to identify—and valuable as farmland. When the lake lay over the land, the silt brought into it by wind and water slowly settled, adding only a fraction of an inch each year to its bed but continuing century after century until the lake bottom was covered dozens of feet deep and every irregularity was hidden. When the water went at last, it revealed a plain as flat as a floor and stretching to the horizon in every direction. It is a land almost without trees and one completely monotonous to anyone raised among even modest hills, but it is a joy to the farmers who work its deep, rich, stoneless soil.

The effects of the ice on life were profound. It completely annihilated everything unable to get out of its slow-moving way, and when it retreated it left a lifeless desert. However, it is very doubtful that any large expanse of barren debris was exposed at any one time by the melting ice because plants can spread with surprising speed even on unlikely soil, and the retreat of the glacier, only a fraction of a mile a year, was no faster than most plants can follow.

The large mammals associated with the ice age have disappeared, some long ago, a few outlasting all four glaciations. The Kodiak bear of Alaska is a splendid survival but the rest are gone.

One of the most curious of the survivals is an insect, the White Mountain butterfly. It is a creature that can exist only in a cold climate and nourishes today in Labrador. It moved south ahead of the glacier and followed it back north as it melted, all except some which sought refuge from the warming climate by fluttering up mountainsides. Two such colonies survive in the United States. One found a safe haven on the chilly slopes of Mount Washington, New England’s highest peak; the other lives on a mountain in Colorado.

Probably the greatest of all effects of the last glacier was that it brought man to North America. Until that time, the Western hemisphere had been, as far as all evidence available to us indicates, empty of humans; with so much water tied up in the icecaps, ocean levels dropped three to four hundred feet to make the Bering Strait a dry and easy passage for nomads from Asia. The first comers very likely were restricted to the Arctic at first, prevented by the ice from moving south. Strange though it may seem, the northern two thirds of Alaska was free of ice except on the mountains, while at the same time the Arctic Ocean was unfrozen. Though hardly balmy, the region was much milder than it is today, and artifacts found along the northern seashore indicate that man tarried there until the glaciation had passed its peak and a way had opened to the south.

But once started, the immigrants came fast. Charred bones of an extinct bison, associated with arrowheads and indicating a feast after a successful hunt, have been found in Clovis, New Mexico; radioactive carbon dating gives their age as approximately 9,900 years. Much more surprising was the debris from another site, the burned bones of a sloth and an extinct horse uncovered near the Strait of Magellan, about as far from the Bering Strait as it is possible to get. Carbon 14 dating established that this meal was eaten nine thousand years ago. The glacier reached its southern limit eleven thousand years ago, and if we accept the idea that man did not find a way south until somewhat later, it means that human beings made their way over rivers and mountains and through jungles from Alaska to the limits of South America in less than two thousand years.

Since the Northern hemisphere has had four separate glaciations in recent geological times, the inevitable question is whether we can expect a fifth. The answer appears to be in the affirmative.

Until recently, attempts to explain the recurring times of ice have assumed a temporary cooling of the earth’s climate for one reason or another. None of these theories has quite explained all the facts. Within the past several years, however, a new theory has been advanced and has gained wide acceptance. Its authors are Maurice Ewing, a geophysicist at Columbia University, and William Bonn, a geologist-meteorologist; their argument is that glacial periods result from periodic warming of the Arctic Ocean.

The Arctic Ocean, they point out, is a sea almost separate from the other waters of the earth. Its connection with the Pacific at the Bering Strait is so narrow and shallow it allows no significant interchange of waters. The passage between the Arctic and the North Atlantic is broader, but across its bottom, between Norway and Greenland, extends a shallow sill, less than three hundred feet deep in most places.

The currents flowing across the sill bring warm Atlantic water into the polar sea, and although the net gain each year is tiny, over thousands of years it is enough to make the Arctic Ocean very much warmer. As a result, the ice on the water becomes thinner, patches of open water grow larger, and eventually the time comes when the ocean is completely free of ice.

A polar sea without ice opens a new stage in the glacial cycle. The warm, open water gives off a great deal of water vapor by evaporation; the moisture is swept south and overland by the winds where it cools off and falls as rain or snow. The open Arctic is such a prolific producer of precipitation that the increased winter snowfall amounts to more than the oblique rays of the sun can melt away during the short northern summer. The snow accumulates and packs into ice until, after tens of thousands of years, the ice cap has become two miles or so thick. The day comes when the tremendous pressure begins to push the ice outward and another continental glacier is on its way.

But even as they form, icecaps carry the mechanism for their own destruction—according to the Ewing-Donn theory. With so much moisture becoming locked up in ice, ocean levels slowly drop until the Bering Strait becomes dry land and the flow over the Norway-Greenland sill becomes greatly restricted, so that not enough warm water can flow into the polar ocean to keep it from freezing. The Arctic Ocean cools off and finally freezes over again. Since very little moisture evaporates from ice, the snowfalls become much lighter—and far to the south the glacier front comes to a stop.

Once again the situation is changed. With snowfall reduced, the glacier is not replenished at the old rate, and so begins to shrink. Water from the melting ice makes the oceans rise, only a fraction of an inch a year but, in the fullness of time, enough to let the currents increase their flow over the northern sill, bringing ever more warm water into the gelid Arctic.

In this year 1960 A.D., we are (still according to the Ewing-Donn theory) at the point in the cycle where the Arctic Ocean is almost ready to shed its old, old, covering of ice. The north-polar ice is going very fast; it covers twelve per cent less area and is forty per cent thinner than it was only fifteen years ago. At that rate it could disappear completely in less than a human lifetime, although we cannot say whether this is a normal climate fluctuation or a steady trend.

But even making allowances, the beginning of the next glacial era might still be breathing down our necks. Eventually the ice will pile up high enough to begin pushing outward from its own weight and will make its slow but inevitable way into the United States again. How long will it be? Much time and study and additional evidence will be necessary before even an informed guess can be made. But we do know one thing: the beginnings will be completely undramatic. Winter snowfalls will increase, so that at the end of the summer some snow will still linger in the tundra lands of northern Canada, probably nothing more than a few patches of slush in the shade of rocks and hummocks of moss. The next autumn the bits of slush will be just a little larger and deeper when the freeze-up comes.